196,441 research outputs found
Thermoelasticity, cation exchange, and deprotonation in Fe-rich holmquistite: Toward a crystal-chemical model for the high-temperature behavior of orthorhombic amphiboles
No full textThe thermoelastic behavior of a crystal of Fe-rich holmquistite with crystal-chemical formula A(K0.01Na0.01)B(Li1.88Mg0.10Na0.02)C(Mg1.68Fe1.422+ Mn0.022+ Al1.88)TSi8.00O22W[(OH)1.97F0.03] was studied by single-crystal X‐ray diffraction at temperatures up to 1023 K, where isothermal annealing in air for 160 h yielded the loss of 0.85 H apfu coupled with oxidation of M1Fe. A complex pattern of cation exchanges was deciphered by comparing structure refinements done before and after annealing. Li migration from the M4 to M3 site is responsible for nonlinearity of the c parameter around 600 K during the first annealing. Cooling of the partially deprotonated crystal to room temperature (RT) showed discontinuities in trends of the b and c parameters around 820-800 K, which cannot be ascribed to a phase transition and can be explained by a rearrangement of the structural units affecting the geometry of the M4 polyhedron. Such discontinuities have never been observed in amphiboles before and must be related to dimensional constraints deriving from the peculiar composition of this amphibole, which contains the smallest possible homovalent constituents, i.e., BLi, CAl, and TSi. The calculated thermoelastic parameters are: Fe-rich holmquistite: αa = 1.36(2)×10-5; αb = 0.55(1)×10-5; αc = 1.5(1)×10-5 - 6.7(9)×10-9; αV = 3.5(3)×10-5 - 0.8(3)×10-8 (polynomial); 2.58(6)×10-5 (linear); partially deprotonated Fe-rich holmquistite: αa = 1.324(9)×10-5 (RT-1023 K); αb = 0.60(1)×10-5 (RT-773 K); αc = 0.68(2)×10-5 (RT-773 K); αV = 2.59(2)×10-5 (RT-773 K). Fe-rich holmquistite is much stiffer than all the previously studied orthorhombic Pnma and Pnmn amphiboles. The results of this work allow progress toward a general model that may explain how the amphibole structure responds to non-ambient conditions, and allows the release of water in diverse geological environments
Fluoronyboite from Jianchang (Su-Lu, China) and nyboite from Nybo (Nordfjord, Norway): A petrological and crystal-chemical comparison of these two high-pressure amphiboles
No full textB-cell lymphomagenesis and human autoimmune models.
No full textAbstract
The concept that B-cell lymphomagenesis represents a multistep process is widely accepted. Pathogenetic events should be better defined both in early and late stages of lymphoproliferation. In the past few years, novel study approaches have been focused on understanding the mechanisms of lymphomagenesis. In particular, immune stimulation by infectious agents or autoantigens, T-cell help, altered immunocompetence, and local cytokine networks seem to be crucial in favouring B-cell expansions. In turn, actively proliferating B cells are at higher risk of undergoing genetic alterations that make the clone capable of fully autonomous growth, i.e., fully neoplastic. Peculiar human autoimmune diseases predisposing to B-cell lymphoma represent relevant models to characterize and dissect the temporal sequence of the different lyphomagenetic events. The present review, particularly, focuses on Sjoegren's syndrome, and on recent findings regarding the putative role of hepatitis C virus in B-cell lymphoproliferation. The biologic and clinical implications may be of major relevance for other B-cell disorders characterized by higher prevalence and morbidity
Crystal structure of adamite at high temperature
No full textStructural modifications with temperature of adamite, Zn-2(AsO4)(OH), were determined by single-crystal X-ray diffraction up to dehydration and collapse of the crystal structure. In the temperature range 25-400 degrees C, adamite shows positive and linear expansion. Axial thermal expansion coefficients, determined over this temperature range, are alpha(a) = 1.06(2) x 10(-5) K-1, alpha(b) = 1.99(2) x 10(-5) K-1, alpha(c) = 3.7(1) x 10(-6) K-1 and alpha(V) = 3.43(3) x 10(-5) K-1. Axial expansion is then strongly anisotropic with alpha(a):alpha(b):alpha(c) = 2.86: 5.38 : 1. Structure refinements of X-ray diffraction data collected at different temperatures allowed us to characterize the mechanisms by which the adamite structure accommodates variations in temperature. Expansion is limited mainly by edge sharing Zn(2) dimers along a and by edge sharing Zn(1) octahedra chains along c; on the other hand, connections of polyhedra along b, the direction of maximum expansion, is governed by corner sharing. Increasing temperature induces mainly an axial expansion of Zn(1) octahedron, which becomes more elongated, and no significant variations of the Zn(2) trigonal bipyramids and As tetrahedra. Starting from 400 degrees C, deviation from a linear evolution of unit-cell parameters is observed, associated with some deterioration of the crystal, a sign of incipient dehydration. The process leads to the formation of Zn-4(AsO4)(2)O
The crystal structure of beryllonite from the type locality and comparison with isopointal structures
No full textThe crystal structure of beryllonite, NaBePO4, has been re-evaluated based on high-resolution X-ray diffraction data using a single crystal from the type locality, Stoneham (Maine, USA). Beryllonite is monoclinic, with a = 8.1309(2) Å, b = 7.7974(2) Å, c = 14.1918(4) Å, ß = 90.039(1)°, V = 899.76(4) Å3, space group P21/n (no. 14), Z = 12. Full anisotropic crystal-structure refinement converged to an R1 value of 0.0504 based on all 9435 unique reflections in the θ range 2–50°. The crystal structure consists of a framework of PO4 and BeO4 tetrahedra sharing all vertexes with adjacent tetrahedra. On the (010) plane, each tetrahedron shares three vertexes with three different tetrahedra forming a quasi-ditrigonal 63 net with alternating PO4 and BeO4 tetrahedra. The remaining vertex of each PO4 and BeO4 tetrahedra points outside the (010) plane and allows corner-linkage among the tetrahedral sheets. Na-centred polyhedra with six-and nine-fold coordinations are located in channels defined by the framework of tetrahedral rings. Beryllonite is isostructural with several natural and synthetic compounds and for these, a quantitative comparison of the crystal-chemical features is reported
Synthetic potassic-ferro-richterite: 1. Composition, crystal structure refinement and HT behavior by in operando single-crystal X-ray diffraction
No full textThe high-temperature behavior of synthetic potassic-ferro-richterite was addressed to obtain data relevant to understanding petrogenetic processes as well as to testing complementarity and mutual calibration of single-crystal X-ray diffraction (XRD) analysis plus structure refinement (SREF) with single-crystal FTIR spectroscopy. This experimental approach aims at: (1) better quantifying the onset of deprotonation, its development and the amount, if any, of relict OH at the end of the process; (2) verifying whether or not the process is homogeneous within the crystal; and (3) evaluating local changes in cation environments close to the OH dipole. In this first part of a series of two papers, we report on the crystal-chemical characterization of potassic-ferro-richterite and on a single-crystal XRD study at high T. Detailed analysis of the available data allowed us to obtain a full characterization of the bulk and crystal chemistry of the studied crystal, hence improving the unit formula suggested by Redhammer & Roth (2002). In operando HT measurements up to 1073 K showed quite anomalous behavior with respect to pargasite/kaersutites, specifically a much lower T for the onset of the deprotonation process (around 500 K), and strongly anomalous behavior of the b angle, which shows inverse slopes for protonated and deprotonated phases. "Oxo-potassic-ferro-richterite" is formed upon deprotonation and remains stable at least up to 1073 K under the conditions of this study. Structure refinements from data collected at different temperatures allowed us to detect changes in the crystal-structure geometry and in turn to decipher the way in which amphiboles with such a peculiar composition respond to increasing T and deprotonation. The thermal expansivity coefficients a(X10∼5 K-1) are: potassic-ferro-richterite: aa= 1.30(6), ab = 0.93(6), ac = 0.12(3), a =-0.49(5), aasinp= 1.34(8), aV = 2.59(2); oxo-potassic-ferro-richterite: aa = 1.71 (3), ab = 0.97(1), ac = 0.193(8), a.p = 0.22(1), aasinii = 1.59(4), aV = 2.74(2)
The crystal structure of beryllonite from the type locality and comparison with isopointal structures
No full textThe crystal structure of beryllonite, NaBePO4, has been re-evaluated based on high-resolution X-ray diffraction data using a single crystal from the type locality, Stoneham (Maine, USA). Beryllonite is monoclinic, with a = 8.1309(2) Å, b = 7.7974(2) Å, c = 14.1918(4) Å, ß = 90.039(1)°, V = 899.76(4) Å3, space group P21/n (no. 14), Z = 12. Full anisotropic crystal-structure refinement converged to an R1 value of 0.0504 based on all 9435 unique reflections in the θ range 2–50°. The crystal structure consists of a framework of PO4 and BeO4 tetrahedra sharing all vertexes with adjacent tetrahedra. On the (010) plane, each tetrahedron shares three vertexes with three different tetrahedra forming a quasi-ditrigonal 63 net with alternating PO4 and BeO4 tetrahedra. The remaining vertex of each PO4 and BeO4 tetrahedra points outside the (010) plane and allows corner-linkage among the tetrahedral sheets. Na-centred polyhedra with six-and nine-fold coordinations are located in channels defined by the framework of tetrahedral rings. Beryllonite is isostructural with several natural and synthetic compounds and for these, a quantitative comparison of the crystal-chemical features is reported
Structure refinement and new crystal-chemical data for tiragalloite (Mn2+ 3.86Ca0.10)σ3.96(As5+ 0.85V5+ 0.02Si0.19)σ1.06 Si3O12(OH) from the Scerscen glacier, Val Malenco, Italy
No full textA new sample of tiragalloite from the Scerscen Glacier of the Malenco valley (Lombardy, Italy) has been studied by single-crystal X-ray diffraction and microchemical analyses. Structure refinement in space group P21/n converge to R1=0.0373 for 6149 reflections with I ≥ 2 Σ(I) and 0.0583 for all 8242 data. The refined unit-cell parameters are: A=6.6702(2) Å, b=19.9336(7) Å, c=7.5759(2) Å, β=95.518(1)°, V=1002.63(5) Å; and the crystal-chemical formula is (Mn2+ 3.86Ca0.10)σ3.96(As5+ 0.85V5+ 0.02Si0.19)σ1.06 Si3O12(OH), with Z=4. The new data of tiragalloite from Scerscen confirm the general organization of the crystal structure, as previously reported in literature for two samples from Graveglia valley, Liguria, Italy, which contained a different amount of V5+ or As5+ cations. However, refinement of the site populations at the Mn(3) and Mn(4) sites suggests that the distribution of Ca among these atom sites might be different than that reported in literature. In particular, in tiragalloite from Scerscen, Ca seems to be preferentially located at the [6]Mn(4)(O5OH) octahedron rather than within the large [7]Mn(3)O7 polyhedron. The topology of the [AsSi3O12(OH)]8- unit of tiragalloite is compared with natural and synthetic phases containing similar groups of four tetrahedra
Aschamalmite (Pb6Bi2S9): crystal structure and ordering scheme for Pb and Bi atoms
No full textThe first single-crystal structure refinement of aschamalmite (Pb6Bi2S9) from Susa Valley (Piedmont, Italy) is reported. The mineral is monoclinic, C2/m, a = 13.719(1) Å, b = 4.132(1) Å, c = 31.419(3) Å, β = 90.94(1)°, V = 1 780.8(4) Å3, Z = 4. The Pb6Bi2S9 compound crystallizes also in an orthorhombic form as heyrovskyite (Cmcm) and our study is focused on understanding the reason leading to a change in symmetry. The aschamalmite structure forms because of ordering between Pb and Bi on the margins of the two octahedral layers that are symmetrically equivalent in heyrovskyite. The two alternate set of octahedral slabs are not related by a crystallographic mirror plane and the symmetry decreases to monoclinic. The cation ordering couples opposite sequences of Pb and Bi octahedra at the margins of slabs. In particular, the succession [Me4A]Bi-[Me5A]Pb-[Me4A]Bi-[Me5A]Pb faced to the series [Me4B]Pb-[Me5B]Bi-[Me4B]Pb-[Me5B]Bi occurs in about 70% of the unit-cells of the crystal, while the contrary sequence ([Me4A]Pb-[Me5A]Bi-[Me4A]Pb-[Me5A]Bi faced to [Me4B]Bi-[Me5B]Pb-[Me4B]Bi-[Me5B]Pb) occurs in the remaining unit-cells. The marginal octahedra have ideal populations (a.p.f.u.): [Me4A]1.40Bi+0.60Pb, [Me4B]1.40Pb+0.60Bi, [Me5A]1.40Pb+0.60Bi, [Me5B]1.40Bi+0.60Pb, in agreement with our structure-refinement results.
The probable site populations for pure heyrovskyite have been proposed, as well as the reasons that prevent the formation of a completely ordered monoclinic phase
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